Arrangement for adjustable volume control



21, 1967 HANS-JURGEN BOBZIN 3,305,792

ARRANGEMENT FOR ADJUSTABLE VOLUME CONTROL Filed June 18, 1963 2 Sheets-Sheet l 6 2 3 5 1'" p4 15 I g 8 1 23 33 21 1s 2s E 5W0 9 22 15 30 32 )3 T T? 27 T T T F IG.1 -7- sgll 49 551' v 53 1.

I V V I T FIG.2

I N VEN TOR HANS 'JURGEN BOBZIN AGE 21, 1967 HANS-JURGEN BOBZIN 3,305,792

ARRANGEMENT FOR ADJUSTABLE VOLUME CONTROL Filed June 18, 1963 2 Sheets-Sheet 2 5/ F/G. 5 g 53 52 62 63 i T .L INVENTOR 56a 75 HANS- JURGE/V 5052/ United States Patent P i 8 Claims. (Cl. 330-133) The invention relates to an arrangement for adjustable volume control, in particular of a sound signal, in which the effective signal is supplied to the output through at least one controllable amplifier element and in which the control quantity, in particular a control direct voltage, for at least one of the amplifier elements is derived by rectification of the oscillations of the eifective signal.

Such an arrangement is of particular importance for television receivers. It has been found that for a good intelligibility of the speech when recording television transmissions in general a considerably higher minimum sound intensity is desired than for purely acoustical transmissions. This may be due to the fact that the absorption capacity of the viewers for acoustical signals is decreased if pictures (optical signals) are simultaneously carefully viewed. Perhaps, also the line frequency radiated by a television receiver in the proximity of the upper frequency limit of the range of audibility has an influence.

If the minimum sound intensity is adjusted higher, a corresponding increase is obtained for those acoustical signals which are transmitted with larger amplitudes. However, this may be quite disturbing, for example when switching from speech to music: since in the transmission of music the maximum sound intensity exceeds the minimum sound intensity considerably more than when trans-.

mitting speech, and a considerably larger total sound intensity may be obtained in the case of music, even if the minimum sound intensity is unchanged.

These differences in sound intensity may be compensated in known manner by volume compression. When using a circuit arrangement which is suitable for the purpose, the amplification, in case of larger sound intensity of the original signal, is decreased by the control quantity so that the sound intensity reproduced increases considerably less.

In case of sound entertainment not accompanied with a picture transmission, the optimum minimum sound intensity is in general determined by external conditions, for example the back-ground noise or the distance and the subjective sensitivity of the listener. If a variation of the total sound intensity is desired, it is in general not necessary to vary the minimum sound intensity, but a control of the maximum sound intensity is sufiicient; the ratio between maximum sound intensity and minimum sound intensity is referred to as volume. By adjustment of the volume, in particular by an edjustable volume com- 7 pression and/or volume expansion, it is consequently possible to achieve practically an adjustment of the sound intensity perceived, and the advantage is obtained that the reproduction of sounds which are mellow at the recording end remains substantially unvaried.

For a ready perceptibility of the mellow tones in the case of low total sound intensityand for minimum disturbance of third parties, for example neighbours-a high degree of volume compression is desirable. On the other hand, volume expansion may be desirable for highfidelity reproduction of musical entertainment, in particular orchestral performances, which are transmitted by the'transmitter with a decreased volume, in which the transition between the two types of volume variation oc- 3,305,792 Patented Feb. 21, 1967 curs continuously and the switching required in known arrangements is to be avoided.

In an arrangement of the type described in the introducttion this i achieved if, according to the invention, a control quantity is produced which causes both a volume compression and a volume expansion, and if the ratio of the actions of the two control quantities is adjustable by moving the sliding contact of a potentiometer. If required, the minimum sound intensity itself also may be adjustable preferably by a potentiometer provided at the output of the volume control arrangement.

It may also be advantageous to permit a substantially complete reduction of the intensity of the output signal possible with the same control device with which the degree of the volume variation is adjusted. As a result of this, the sound intensity at maximum volume compression may be substantially reduced to zero so that an adaptation of the total sound intensity to a given low value is possible without having to use a separate control device.

This result is obtained ifan auxiliary signal of constant amplitude is applied to the part of the circuit which supplies the control quantity for the volume compression. The auxiliary signal has an amplitude such that when sliding the contact is moved to the end of the potentiometer which corresponds to maximum volume compression, the output signal is substantially entirely suppressed.

In order that the invention may readily be carried into effect, several embodiments thereof will now be vdescribed more fully, by way of example, with reference to the accompanying drawing, in which:

FIG. 1 shows an embodiment in which the control quantity for volume compression and for volume expansion is supplied to different stages.

FIG. 2 shows an embodiment in which only one stage need be controlled.

FIG. 3 illustrates a modification of a part of the circuit of FIG. 1, FIG. 4 illustrates a modification of a part of the circuit of FIG. 2, and FIG. 5 illustrates another modification of a part of the circuit of FIG. 2.

Referring now to FIG. 1 the input signal, for example a sound signal, is applied from a terminal 1, through a blocking capacitor 2, to a controllable amplifier pentode 3. The screen grid pentode 3 is connected to the positive terminal of, for example, +200-volt of the supply source and the cathode and the suppressor grid of this tube are connected to the grounded negative terminal of the supply source. The anode is connected, through a load resistor 4, to the positive terminal of the source and, through a blocking capacitor 5, to the control grid of a second controllable amplifier pentode 6. The screen grid of pentode 6 is connected to the positive terminal and the suppressor grid is connected to the cathode. The anode is connected, through a load resistor 7, to the positive terminal of the supply source and, through a blocking capacitor 8, to an output potentiometer 9 which is connected to ground at its other end. Output oscillations derived from the sliding contact of the potentiometer are applied to the terminal 10. The cathode of the tube 6 which is connected to ground by the capacitor-13 for signal oscillations, is connected to a potentiometer 11, 12 A v the other rectifier circuit supplies a control quantity increasing the amplification of the succeeding amplifier element, the tube 6.

For that purpose, the end of potentiometer 16 shown in the drawing on the left-hand side is connected to a resistor 17 which is connected to ground at its other end, and to the cathode of a first diode 18. The anode of this diode is connected to ground through a resistor 19 and a charging capacitor 20 connected in parallel therewith. A negative control direct voltage is derived from the anode of the diode 18 through a smoothing member which consists of a series resistor 21 and a by-pass capacitor 22, and this voltage is applied to the grid of the tube 3 through a series resistor 23.

The other end of the potentiometer 16 is connected to a resistor 27 which is connected to ground at its other end and to the anode of a second diode 28. The cathode of this diode is connected to ground through a resistor 29 and a charging capacitor 30 connected in parallel therewith. A positive control direct voltage which is derived from the cathode of the diode 28 through a smoothing member which consists of a series resistor 31 and a by-pass capacitor 32, is supplied to a grid of the tube 6 through a series resistor 33.

The circuits with the diodes 18 and 28 are preferably proportioned equally. The resistances of resistors 17 and 27 respectively are preferably considerably smaller than the resistance of the potentiometer 16 so that the signal voltage at the resistor 27 is only a few percent or less of the signal voltage at the resistor 17 if the sliding contact of the potentiometer 16 is moved to the left; similar conditions occur at the resistor 17 if the sliding contact is moved entirely to the right.

The resistance value of the rectifier circut should in any position of the sliding contact at the potentiometer 16 considerably exceed the value of the load resistor 4 of the tube 3 in order that a movement of the sliding contact does not produce a disturbing influence on the amplification between the terminals 1 and The rectifier circuits preferably operate as point-contact rectifiers; the charging time constants for the control voltages at the capacitors and 30 should correspond to the cycle of the highest frequency signal oscillations to be transmitted. The discharge time constant of the rectifier circuits, in particular of the capacitors 22 and 32, should be large as compared with the cycle of the lowest frequency signal oscillations to be transmitted, in the case of sound signals approximately 3-30 seconds, preferably 7-15 seconds. Good results are obtained with charging time constants in the order of magnitude of from 0.05-O.4 second, preferably at 0.2 second.

The load resistors 19 and 29 should be larger, preferably considerably larger than the resistors 17 and 27.

The operation of the circuit described so far is as follows:

The signal voltages occurring at the output of the tube 3 are supplied to the potentiometer 16 through the capacitor 15. At the two ends of this potentiometer fractions of this signal voltage occur at the resistors 17 and 27 which are dependent upon the position of the sliding contact at the potentiometer 16. The components of the circuit are chosen so that when the sliding contact is approximately in the center, and the amplitude of the signals is high, only negligible alternating voltages occur at the resistors 17 and 27 and consequently at the rectifier circuit, so that the bias voltages of the tubes 3 and 6 are at least substantially constant. Therefore volume compression does not occur in the tube 3 and volume expansion does not occur in tube 6. Since the influences of the two tubes are otherwise opposite to one another, they practically neutralize each other even if considerable voltages occur at the two resistors 17 and 27 when the sliding contact is approximately in the center of the potentiometer 16 so that in this case the signal from the terminal 1 to the terminal 10 is transmitted substantially without volume variation.

If the sliding contact is positioned at the left-hand end of the potentiometer 16 the entire alternating voltage is transmitted from the capacitor 15 to the resistor 17 and rectified and produces a negative bias voltage for the tube 3.

This has substantially no influence in the case of weak signals; in the case of strong signals, however, the amplification of the tube 3 is substantially decreased thereby, so that such signals are transmitted with considerably decreased amplitude to the tube 6 and consequently to the output terminal 10: consequently volume compression occurs.

If the sliding contact is positioned at the right-hand end of the potentiometer 17 the entire alternating voltage is transmitted from the capacitor 15 to the resistor 27 and rectified and produces a positive bias voltage for the grid of the tube 6. The cathode of the tube 6 is biased positively to such an extent by the potentiometer 11, 12 that, normally, it produces only a slight amplification. If the sliding contact of the potentiometer 16 is positioned at the right-hand end, the operating point of the tube 6 is shifted to larger amplification values by the positive control voltage occurring. In the case of small signal amplitudes, this again has hardly any influence; in the case of strong signals, however, the amplification of the tube 6 is substantially increased so that such signals reach the output terminal 10 with a considerably increased amplitude: consequently volume expansion occurs.

By sliding the sliding contact of the potentiometer 16, the adjustment may continuously be varied between a maximum value of volume compression and a maximum value of volume expansion.

The value of the lowest signal strength, at which in no position of the sliding contact at the potentiometer 16 a considerable amplification variation of one of the tubes is produced, so that an unvaried transmission occurs, is determined on the one hand by the proportioning of the circuit elements and may be reduced with the sliding contact at the potentiometer 9.

It may also be desirable to control the output signal strength to zero during maximum volume compression without the use of an additional control member, if the sliding contact of the potentiometer 16 is moved to the left-hand end connected to the resistor 17. This is possible if, when moving the volume control member over the point corresponding to maximum compression, an auxiliary control quantity which substantially entirely cuts off the transmission channel, for example an amplifier element, is rendered operative gradually.

In order to achieve this an alternating voltage auxiliary signal, the frequency of which preferably lies outside the frequency range of the eifective signal, may be supplied from a terminal 37 (see FIG. 3) to the sliding contact of the potentiometer 16 through a blocking capacitor 36. This auxiliary signal preferably consists of narrow pulses as supplied, for example, by the line deflection device of a television receiver, and the polarity is such that the pulses open the rectifier 18 which supplies the control quantity for the volume compression. Such pulses are shown at 38 at the terminal 37.

The auxiliary signal is consequently superimposed on the effective signal.

If the sliding contact of the potentiometer 16 is approximately in the central position, the effective signal reaches values which may be considerably larger than the amplitude of the auxiliary signal. The influence of the auxiliary signal on the circuit consequently is small. When moving the sliding contact to the cathode of the diode 18, the voltage at the resistor 17 increases, so that an increasingly larger control voltage is produced for the tube 3. As a result of this the amplification for the effective signal decreases and its volume is decreased. When moving the sliding contact of the potentiometer 16 further to the left, so much negative control voltage is finally produced that even the highest values of the effective signal at the capacitor 15 and consequently at the sliding contact of the potentiometer 16 no longer reach the amplitude of the auxiliary signal 38: therefore the control voltage is produced for a considerable part by the auxiliary signal 38 only and is available also in the case of the smallest input signal; as a result of this, the total amplification, as well as the amplitude of weak eflective signals, is decreased. The amplitude of the auxiliary signals is preferably chosen large so that when moving the sliding contact towards the end of the potentiometer 16 corresponding to maximum volume compression so large a negative control voltage is produced by the rectifier arrangemen-t 18-22 that the tube 3 is substantially entirely cut off and consequently the output signal is suppressed. In this manner the sound intensity may be cont-rolled to provide Zero output.

It appears that, with the type and polarity of the auxiliary signal 38 described, the positive parts of the effective signal which are decisive of the volume expansion across the rectifier 28 are substantially unaltered, so that the auxiliary signal has substantially no influence on the volume expansion. If required, for example, if a sinusoidal auxiliary signal is used, a suppressor 40 for the auxiliary signal, for example a choke coil, may be connected before the rectifier 28.

In a corresponding manner the signal channel may be separated from the auxiliary signal by connecting a suppressor 41 between the sliding contact of the potentiometer 16 connected to the auxiliary signal source and the capacitor 15 supplying the elfective sign-a1 oscillations. Instead of a choke coil, any other suppressing network, for example a parallel resonant circuit, may be used.

FIG. 2 shows another embodiment of the invention in which only one controlled amplifier stage need be used.

In this circuit, the input oscillations are supplied from terminal 1 through a blocking capacitor 2 to a controlled pentode 45. The screen grid of tube -45 is connect-ed to the positive terminal of the supply source and the suppressor grid is connected to the cathode. A load resistor 46 is connected in the anode branch. The anode is also connected through a blocking capacitor 8 to the output potentiometer 9, which is grounded at its other end. The sliding contact of the potentiometer 9 is connected to output terminal 10. The cathode of the tube 45 is connected to the tapping of a potentiometer connected to the supply source. This potentiometer consists of the resistors 47 and 48. The cathode is further connected to ground for the signal oscillations through the capacitor 49. The potentiometer 47, 48 is adjusted so that the cathode voltage of the tube 45 is at least substantially constant independent of the grid bias and consequently of the operating point adjustment. If desired, a possibly desired dependence on the operating point may be allowed by higher values for the resistors 47 and 48; as a result of this, the control range may be varied by the resultant direct current negative feedback.

The input oscillations of the terminal 1 are also supplied through a blocking capacitor 50 to the anode of a rectifier 51, the cathod of which is connected to ground through a charging capacitor 52, and, if desired, a load resistor 53. The direct current circuit is closed by a resistor 54 between the anode of the rectifier 51 and ground. The end, on the left-hand side in the drawing, of a potentiometer 56 is connected to the cathode of the rectifier 51.

A second rectifier 61 has its anode connected to ground through a charging capacitor 62, and, if desired, a load resistor 63. The cathode of rectifier 61 is connected to the anode of the tube 45 through a blocking capacitor 60. The direct current circuit is closed :by a resistor 64 connected between the cathode of the rectifier 61 and ground. The anode of the rectifier 61 is connected to the other (right-hand) end of the potentiometer 56.

The control grid of the control tube 45 is connected to the sliding contact of the potentiometer 56 through a smoothing member, which consists of a series resistor 65 and a by-pass capacitor 66, and through a series resistor 67.

As in the circuit shown in FIG. 1, the diodes 51 and 61 operate as point-contact rectifiers having small charging time constants and considerably larger discharge time constants. The resistor of the potentiometer 56 should preferably be large with respect to the resistors 53 and 63 in order to avoid any undesired coupling between the two rectifier circuits. The load of the input oscillations at the terminal 1 and of the output oscillations at the anode of the tube 45 by the circuits with the diodes 51 and 61 should be substantially negligible and should preferably not cause any interfering variation of the frequency characteristic.

A positive direct voltage is produced by the rectifier circuit 51 at the left-hand end of the potentiometer 56. This voltage fluctuates with the amplitude of the input oscillations averaged through a small period of time. Ac cordingly, by the action of the diode 61 a negative direct voltage is obtained at the right-hand end of the potentiometer 56, which fluctuates with the amplitude of the output oscillations averaged through a small period of time.

Consequently, a control quantity suitable -for volume expansion is supplied at the left-hand end of the potentiometer 56 and a control quantity suitable for volume compression is supplied at the right-hand end of this potentiometer 56, and the control quantity for the amplifier stage with the tube 45 is derived from the sliding contact. Consequently, maximum volume expansion is obtained when the slider is at the extreme left-hand position, and maximum volume compression is obtained when the slider is at the extreme right-hand position. In a central position a transmission is obtained between the input 1 and the output 10 which is independent of the amplitude and unvaried in volume.

It the tube 45 has high amplification with large input signal amplitude and the potentiometer is adjusted for volume expansion, a substantially higher direct voltage may appear at the capacitor 62 than at the capacitor 52. If desired, this may be avoided by supplying to the rectifier circuit with the diode 61 the signal voltages through a potentiometer which decreases said voltages correspondingly. In practice, this is usually not required, however, since in case of a suflicient value of the potentiometer 56 the voltage of the capacitor 62 does not substantially influence that at the capacitor 52 when adjusting at volume expansion; on the other hand, when adjusting at volume compression (sliding contact in the proximity of the righthand end of the resistor 56) a reducing effect occurs so that it is desirable to supply in this case the ouput amplitude to the diode 61 to the fullest possible extent.

For the case, in which the grid potential approximately corresponds to ground potential, the operating point of the amplifier tube 45 may be adjusted at a central portion of the characteristic by the potentiometer 47, 48.

In order that a control of the output amplitude down to the value of zero occurs by moving the sliding contact of the potentiometer 56, it is possible, as in FIGURE 1, to supply an auxiliary signal, preferably in the form of negatively orientated pulses of high frequency, for example line pulses of a television receiver, as the control quantity for volume compression from a terminal 70 (see FIG. 4) through a coupling capacitor 71 to the cathode of the diode 61 of the rectifier circuit. In order to prevent any interferences in the signal channel, a suppressor for the auxiliary signal, for example a choke coil 72, may be connected between the capacitor 60 and the anode of the tube 45. Between the anode of the diode 61 and the charging capacitor 62, to which also the one end of the potentiometer 56 is connected, a further suppressor for the auxiliary signal, for example a choke coil 73, is

connected. In addition, the anode of the diode 61 is connected to the sliding contact of the potentiometer 56 through an impedance which is well permeable for the auxiliary signal, for example a capacitor 74, which may be replaced, if desired, by a series resonant circuit or any other suitable network.

The impedances 73 and 74 are small as compared with the resistance value of the potentiometer 56 between its end connected to the capacitor 62 and a small part, for example 10-25% of its sliding path. As a result of this, when adjusting the sliding contact remote from the righthand stop member associated with volume compression, the suppressor 73 is operative between the diode 61 and the charge capacitor 62, so that a rectification of the auxiliary signal does not occur and only the control direct voltage corresponding to the effective signal amplitude occurs at the capacitor 62. If the sliding contact of the potentiometer 56 is moved to the right-hand stop member, its resistance value between sliding contact and ground becomes small and the suppressor 73 is bridged together with the impedance of the capacitor 74 which is also small. Then the auxiliary signal also is rectified and produces a control direct voltage by which the tube 45 may be substantially entirely cut off; the effective output signal of the tube 45 no longer cooperates. Consequently, the desired control back to zero is achieved indeed.

Due to the use of the impedances 73 and 74, when adjusting the sliding contact of the potentiometer 56, in particular to the center and the left-hand end, an additional direct voltage is not produced, so that at low input signal the bias voltage of the tube 45 is independent of the position of the sliding contact of the potentiometer 56. Only when moving the sliding contact to the righthand end does the auxiliary signal become operative, by bridging of the impedance 63, in the rectifier circuit and produces the direct voltage necessary for cutting off the tube 45.

Referring to FIG. 2a, in which the control potentiometer 56a is shown with a fixed tap, a corresponding action may also be obtained with an auxiliary direct current signal. Between the tap and the left-hand end, the circuit is substantially unvaried With respect to the corresponding portion of FIG. 2. The right-hand end of the potentiometer, however, is connected to ground, and a positive bias voltage of, for example, 10 volts is supplied to the tap. This voltage accordingly is also operative at the capacitor 62 and at the resistor 63. The cathode of the tube 45 must be connected to a more positive potential by the same amount by proportioning of the potentiometer 47, 48; the same holds also for the rectifier voltage for the volume expansion control quantity with the diode 51. At any rate the fundamental bias supplied may not become operative as a threshold value for the diode; therefore the anode of the diode 51 is to be connected through a resistor 76 to the positive terminal of the source and shown in FIGURE 5. The whole grid circuit arrangement is consequently more positive by a few volts; otherwise, however, no other effect is obtained than in FIG. 2 if the sliding contact is moved between the left-hand end and the tapping of the potentiometer 56a. If the sliding contact is moved further to the right a more negative direct voltage which is independent of the rectifier circuit which the diode 61 is supplied to it, by which the desired cutting off of the tube 45 and consequently control back of the output sound intensity to zero is achieved.

What is claimed is:

1. An adjustable volume control circuit comprising an amplifier having first and second cascade connected controllable amplifier devices each having a control electrode, first and second rectifier circuits, a potentiometer having a movable arm, means connecting said first rectifier circuit to one end of said potentiometer for producing a first control voltage, means connecting said second rectifier circuit to the other end of said potentiometer for producing a second control voltage, whereby the ratio of. said first and second control voltage, depends on the position of said arm, means applying said first control voltage to said control electrode of said first amplifier device for controlling volume compression of signals amplified in said first device, and means applying said second control voltage to said control electrode of said second amplifier device for controlling volume expansion of signals amplified in said second device.

2. The volume control circuit of claim 1 comprising a source of an auxiliary constant amplitude signal of a frequency outside the range of signals to be amplified in said amplifier, means applying said auxiliary signals to the arm of said potentiometer, said auxiliary signals having sufiicient amplitude that when said arm is at said one end of said potentiometer said first control voltage is sufiiciently large to cut-off said first amplifier device.

3. The volume control circuit of claim 2 comprising suppressor means, and means for connecting said suppressor means between said other end of said potentiometer and said second rectifier circuit for preventing application of said auxiliary signal to said second rectifier circult.

4. The volume control circuit of claim 2 wherein said auxiliary signal is a pulsatory signal of a predetermined polarity, said first rectifier circuit being connected to pass said pulsatory signal and said second rectifier circuit being connected to block said pulsatory signal.

5. An adjustable volume control circuit comprising an amplifier having first and second amplifier devices each having input, output and common electrodes, a signal input circuit connected to the input electrode of said first device, a signal output circuit connected to the output electrode of said second device, means connecting the output electrode of said first device to the input electrode of said second device, means for substantially maintaining the voltages at common electrodes of said first and second devices at constant potentials whereby the amplification characteristics of said first and second devices are determined by the direct voltage levels of said input electrodes of said first and second devices respectively, first and second rectifier circuits, a potentiometer having a movable arm, means connecting said arm to the output electrode of said first device, means connecting one end of said potentiometer to said first rectifier device for producing a first control voltage, means for applying said first control voltage to the input electrode of said first device with a polarity to control the compression of amplification in said first device, with maximum compression occurring when said arm is at said first end, means connecting the other end of said potentiometer to said second rectifier device for producing a second control voltage, and means applying said second control voltage to the input electrode of said second device to control the expansion of amplification in said second device, whereby maximum expansion occurs when said arm is at said other end.

6. An adjustable volume control circuit comprising an amplifier having an input circuit and an output circuit and comprising a controllable amplifier device, a first rectifier circuit connected to said input circuit for producing an expansion control voltage, a second rectifier circuit connected to said output circuit for producing a compression control voltage, a potentiometer connected between said first and second rectifier circuits, said potentiometer having an adjustable arm, means connecting said arm to said controllable device for controlling the amplification therein, whereby the ratio of said expansion and compression control voltages applied to said device depends upon the position of said arm.

7. The control circuit of claim 6 comprising a source of an auxiliary constant amplitude signal of a frequency outside the frequency range of signals to be amplified in said amplifier, means applying said auxiliary voltage to said second rectifier circuit, said second rectifier circuit including a charging circuit across which said second control voltage is developed and suppressor means for inhibiting the production of rectified auxiliary signals across said charging circuit, means connecting one end of said potentiometer to said charging circuit, a capacitor and means connecting said capacitor between said arm and said suppressor whereby said capacitor is in shunt with said suppressor when said arm is at said one end, said auxiliary signal having sufficient amplitude that said second control voltage cuts off said device when said arm is at said one end.

8. The control circuit of claim 6 wherein said potentiometer has a fixed tap and the position of said potentiometer between one end and said fixed tap is connected 10 between said first and second rectifier circuits in that order, comprising means connecting the other end of said potentiometer to a point of fixed potential, a source of direct voltage, resistor means for applying said direct voltage to said fixed tap, and means for biassing said device whereby said device is cut-off only when said arm is at said other end.

No references cited.

DAVID G. REDINBAUGH, Primary Examiner.

I. MCHUGH, R. L. RICHARDSON,

Assistant Examiners. 

6. AN ADJUSTABLE VOLUME CONTROL CIRCUIT COMPRISING AN AMPLIFIER HAVING AN INPUT CIRCUIT AN OUTPUT CIRCUIT AND COMPRISING A CONTROLLABLE AMPLIFIER DEVICE, A FIRST RECTIFIER CIRCUIT CONNECTED TO SAID INPUT CIRCUIT FOR PRODUCING AN EXPANSION CONTROL VOLTAGE, A SECOND RECTIFIER CIRCUIT CONNECTED TO SAID OUTPUT CIRCUIT FOR PRODUCING A COMPRESSION CONTROL VOLTAGE, A POTENTIOMETER CONNECTED BETWEEN SAID FIRST AND SECOND RECTIFIER CIRCUITS, SAID POTENTIOMETER HAVING AN ADJUSTABLE ARM, MEANS CONNECTING SAID ARM TO SAID CONTROLLABLE DEVICE FOR CONTROLLING THE AMPLIFICATION THEREIN, WHEREBY THE RATIO OF SAID EXPANSION AND COMPRESSION CONTROL VOLTAGES APPLIED TO SAID DEVICE DEPENDS UPON THE POSITION OF SAID ARM. 